The primary emphasis in genetic modification has been directed to prokaryotes and mammalian cells. For a variety of reasons, plants have proven more intransigent than other eukaryotic cells to genetically manipulate. However, in many instances, it is desirable to effect transcription of an introduced nucleotide sequence of interest either specifically or preferentially in a particular plant part or at a particular developmental stage of the plant. Accordingly, there is substantial interest in identifying transcriptional control sequences, such as promoters or enhancers, which specifically or preferentially direct transcription in particular plant organs, tissues or cell types or at particular developmental stages of the plant.
Expression of heterologous DNA sequences in a plant is dependent upon the presence of an operably linked transcriptional control sequence, such as a promoter or enhancer, which is functional within the plant. The choice of transcriptional control sequence will determine when and where within the organism the heterologous DNA sequence is expressed. For example, where continuous expression is desired throughout the cells of a plant, constitutive promoters are utilized. In contrast, where gene expression in response to a stimulus is desired, an inducible promoter may be used. Where expression in specific tissues or organs is desired, a tissue-specific promoter may be used.
Frequently, it is desirable to effect expression of a DNA sequence in particular cells, tissues or organs of a plant. For example, male and/or female sterility in a plant might be accomplished by genetic manipulation of the plant's genome with a male or female gamete specific promoter operably linked to a toxic protein.
Alternatively, it might be desirable to inhibit expression of a native DNA sequence within particular plant tissues to achieve a desired phenotype. In this case, such inhibition might be accomplished by transformation of the plant with a tissue-specific promoter operably linked to an antisense or RNAi nucleotide sequence, such that expression of these sequences produces an RNA transcript that interferes with translation of the mRNA of the native DNA sequence.
Promoter sequences that can be used to drive egg cell specific expression of a nucleotide sequence of interest in higher plants are not presently available. This may be at least in part attributed to the difficulty in isolating female gametes from seed plants. As a consequence of this difficulty, the transcripts of plant egg cells are poorly represented in current databases of expressed sequence tags (ESTs), which have been mainly generated through sequencing from cDNA libraries produced from complex tissues, e.g. whole floral organs. Though more than 1.5 million Poaceae ESTs were present in the public EST database (by March 2004) the use of complex tissues resulted in under representation of genes expressed at low levels and in only one or a few cell types.
However, the isolation and characterization of egg cell-specific transcriptional control sequences would be desirable for use in the genetic manipulation of plants.
Reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in any country.